Spin-Pair Tunneling in Mn$_3$ Single-Molecule Magnet

We report spin-pair tunneling observed in Mn$_3$ single-molecule magnet, which is a crystal with 2D network of identical exchange coupling. We observed a series of extra quantum tunnelings by the ac susceptibility measurements, and demonstrated these are concerted tunnelings of two spins taking place from the same initial state to the same final state simultaneously. The resonant field of spin-pair tunneling can be expressed as $H_z=lD/g\mu_{0}\mu_{B}+(n_{\downarrow}-n_{\uparrow})JS/2{g\mu_{0}\mu_{B}}$, and the splitting interval ($|J|S/{g\mu_{0}\mu_{B}}$) is half of that of the single-spin tunneling ($2|J|S/{g\mu_{0}\mu_{B}}$), which is analogous to the relationship between the magnetic flux quantum in superconductor ($h/2e$) and common metal ($h/e$).